The ability to control focusing and distribution of light is critical in order to provide efficient illumination. Many optical systems employ one or more optical elements to achieve a desired light distribution and/or desired illumination uniformity. Depending on situation and requirements various light sources are employed, including incandescence, fluorescence and solid state based light sources. The design of the optical system typically varies depending on the light source used. It is however problematic to compensate for spatial and/or intensity variations in intensity and/or colour of the light sources used, which is critical when producing uniform illumination. Typical problems encountered for instance when using an optical system in the form of a standard total internal reflection (TIR) collimator are 1) Colour over angle variation of a light source, leading to colour at different off axis angles in the far field, 2) Colour over position variations of a light source, which also leads to colour artifacts, and 3) Strong non-uniformities in the intensity of the collimator when using an arrays of light sources having dark regions (i.e. regions not producing light) between them.
One way to counteract these problems is to utilize techniques to disperse and redistribute or mix light using diffusion elements or facets on the exit surface on a total internal reflection (TIR) collimator. U.S. 2013/0058103 for example discloses a collimator (or lens as it is called in U.S. 2013/0058103) comprising: a lens body; a total reflection surface provided on an outer side of the lens body, the reflection surface being in the form of a scale like polyhedron; a recess formed on a bottom side of the lens body at a central region thereof for accommodating a LED, the recess having a side surface and a central surface; a micro lens array formed at the central surface of the recess; and a light-emitting surface provided at a top side of the lens body; wherein a substantially uniform circular light spot is formed by the lens.
However, it is challenging to alleviate all the problems mentioned above utilizing the collimator of U.S. 2013/0058103, especially if the light source varies strongly in luminance over position. The faceting and diffusion elements have accordingly to be pronounced in order to mix the light such that the effects pertaining to the in-homogeneity of the light source are smeared out. As a result a uniform illumination may be provided, but at the same time the beam angle of the light leaving the optical system is increased which hinders collimation of the light and reduces control of the light distribution.
Hence there is a need for finding optical systems providing better illumination uniformity and light distribution control.